For a suitcase 11a shown in FIG. 1 with rollers 10a installed at the bottom thereof, there is a retractable draw bar 12a allocated to facilitate the protraction/retraction of a pull handle 13a so that the pull handle 13a can be pulled out to a full extent to be gripped conveniently when used and be retracted to a compact position when not used.
As shown in FIGS. 2 and 3, the retractable draw bar 12a comprises basically a first joint tube 14a, a second joint tube 15a, and a positioning seat 16a. The caliber of the first joint tube 14a is slightly larger than that of the second joint tube 15a so that the second joint tube 15a can be retractably inserted therein. Two circular retaining holes 18a are respectively disposed on two opposite tube walls near an upper opening 17a of the first joint tube 14a. The positioning seat 16a is sheathed in the upper opening 17a of the first joint tube 14a. Two circular retaining buttons 19a are respectively formed on two opposite sides of the positioning seat 16a. The positioning seat 16a can be joined at the upper opening 17a through the fastening of the retaining buttons 19a in the retaining holes 18a. A fixing element 20a can be fixed in the lower opening of the second joint tube 15a. When the second joint tube 15a is pulled upwards in the first joint tube 14a until the fixing element 20a sticks to the positioning seat 16a, the second joint tube 15a can be positioned there so that it will not slide out of the first joint tube 14a.
However, in the above positioning structure of a retractable draw bar in prior art, the positioning seat 16a and the first joint tube 14a are joined through the fastening of the retaining buttons 19a in the retaining holes 18a. When the second joint tube 15a is pulled upwards, the positioning seat 16a will bear an upward force. The retaining buttons 19a and the retaining holes 18a will bear this force via the upper edges thereof. Because the contact area of the upper edges of the retaining buttons 19a and the retaining holes 18a is very small, the retaining buttons 19a will easily break or be damaged because of large pressure. Therefore, the positioning seat 16a can not be steadily joined at the upper opening 17a of the first joint tube 14a so that the positioning seat 16a can not achieve its original positioning function.